The present invention relates to a two-stroke cycle internal combustion engine that is suited for use in, for example, a portable power working machine and, in particular, to a two-stroke cycle internal combustion engine that produces exhaust gases that are cleaner than the exhaust gases emitted by many previously known two-stroke cycle engines and in which blow-by of unburned air-fuel mixture is reduced.
Two-stroke cycle internal combustion engines of the type that are commonly used in portable power working machines, such as bush cutters and chain saws, include a cylinder head that forms with a piston a combustion actuating chamber and a crankcase forming a crank chamber. (Although the chamber formed in the cylinder may be called a combustion chamber, an actuating chamber, a cylinder chamber, etc., the chamber in the cylinder is generically referred to in the present specification as a combustion actuating chamber.) An intake port, a scavenging port and an exhaust port, that are opened and closed by the piston, are provided in the trunk portion of the cylinder. One cycle of the operation of the engine is accomplished by two strokes of the pistonxe2x80x94there is no stroke assigned exclusively to either the intake of fuel/air or the exhaust of combustion gases, as in a four-stroke cycle engine.
During the ascending stroke of the piston, an air-fuel mixture is inducted from the intake port into the crank chamber below the piston. When the piston descends, the air-fuel mixture in the crank chamber is pre-compressed, producing a compressed gas mixture, which is conducted through a scavenging passage and then blown from the scavenging port into the working chamber above the piston so as to push the combustion waste gases toward the exhaust portxe2x80x94the flow of the compressed air/fuel mixture is thus utilized for xe2x80x9cscavengingxe2x80x9d the combustion waste gas. The unburned air-fuel mixture used for scavenging is likely to be mixed with the combustion gas (exhaust gas), thereby increasing the quantity of so-called xe2x80x9cblow-by,xe2x80x9d the quantity of air-fuel mixture discharged without being utilized for combustion. Because of blow-by, two-stroke cycle internal combustion engines are not only inferior from the point of view of fuel consumption but also disadvantageous in that a larger amount of noxious components, such as HC (unburned components of the fuel) and CO (incompletely burned fuel) are present in the exhaust gas as compared with a four-stroke cycle engine. Therefore, even if the two-stroke cycle engine is of small displacement, the contribution of the noxious components to environmental contamination is significant. Additionally, conventional two-stroke cycle internal combustion engines are often not able to meet the increasingly severe exhaust gas emission regulations that are being established by governmental authorities, particularly with regard to the HC components (total HC) in the exhaust gases.
Generally, a mixed fuel consisting of gasoline and a lubricant oil is used in two-stroke cycle internal combustion engines. Therefore, the exhaust gas is also contaminated by the oil component of the fuel. Depending on the operating attitude (inclination) of the engine, moreover, an excessive amount of the oil component of the fuel may be introduced into the combustion actuating chamber, thereby causing problems in the operation of the engine.
The present invention has been made to overcome the aforementioned problems. An object of the present invention is, accordingly, to provide a two-stroke cycle internal combustion engine that emits exhaust gases with lesser amounts of noxious components, such as HC, than contained in the exhaust gases emitted by many previously known two-stroke cycle engines and in which blow-by of unburned air-fuel mixture is reduced. It is also an object to attain such improvements without necessitating a considerable modification in the structure of the engine or a substantial increase in manufacturing cost. Yet another object is to reduce the instances of operational problems with two-stroke cycle engines.
To attain the aforementioned objects, a two-stroke cycle internal combustion engine, according to the present invention, has a crank chamber located below a piston and a combustion actuating chamber located above the piston. Either one pair or two pairs of scavenging passageways communicate the crank chamber with the combustion actuating chamber, the scavenging passageways of the one pair or of each of the two pairs being disposed symmetrically with respect to a plane that bisects an exhaust port so as to form a Schnxc3xcrle-type scavenging system. Each scavenging passageway is throttled at a location proximate to a scavenging inlet port thereof.
In some embodiments of the present invention, the combustion actuating chamber may be defined by a cylinder member, the crank chamber is defined by a crankcase member connected to an underside of the cylinder member, and a plate-like member is interposed between the cylinder member and the crankcase member. The plate-like member has a throttling hole having an open area which is smaller than the cross-sectional area of the scavenging passageway at locations immediately adjacent the plate-like member so as to throttle a portion of each of the scavenging passageways which is located close to the scavenging inlet port.
In other embodiments of the internal combustion engine according to the present invention, a partition wall provided with a throttling hole having an opening area which is smaller than the cross-sectional area of the scavenging passageway is disposed close to the scavenging inlet port of each of the scavenging passageways.
In preferred embodiments of the internal combustion engine according to the present invention, the position and size of each throttling hole are selected in a manner such as to cause a turbulent flow in the scavenging gas flow through the scavenging passageway.
It is also advantageous for an internal combustion engine according to the present invention to include a step portion formed in a region close to the scavenging inlet port of each of the scavenging passageways due to the provision of the throttling hole.
In further preferred embodiments of the internal combustion engine according to the present invention, a scavenging outlet port of each of said pair or pairs of scavenging passageways is also throttled.
In the operation of two-stroke cycle internal combustion engines according to the present invention constructed as described above, as the pressure inside the crank chamber is reduced during the ascending stroke of the piston, an air-fuel mixture supplied from an air-fuel generating device, such as a carburetor, is inducted into and held in the crank chamber. When the air-fuel mixture inside the combustion actuating chamber disposed above the piston is ignited and burns, the piston is driven downwardly due to the generation of combustion gas. Upon the descending stroke of the piston, the air-fuel mixture held in the crank chamber and the scavenging passageways is compressed by the piston. Near the end of the descending stroke, the exhaust port opens first and when the piston has further descended, the scavenging outlet port provided at a downstream end of the scavenging passageway is opened. During the scavenging period where the scavenging outlet port is opened, the air-fuel mixture compressed in the crank chamber flows toward the downstream end of the scavenging passageway toward the combustion actuating chamber and is discharged out of the scavenging outlet port as a scavenging gas flow having a predetermined horizontal scavenging angle directed toward an inner wall portion of the cylinder bore which is located opposite to the exhaust port. The scavenging gas flow subsequently impinges against the aforementioned inner wall portion of the cylinder bore and then is turned back therefrom to thereby push the combustion waste gases out of the exhaust port.
The throttling hole provided in the vicinity of the scavenging inlet port of each of the scavenging passageways causes a relatively large difference in pressure to be generated between the crank chamber and the downstream side of the throttling hole of each of the scavenging passageways, as compared with the case where the throttling hole is not provided, thereby causing the air-fuel mixture to be ejected as a high-velocity jet from the throttling hole and to flow downstream therefrom. Thus, the provision of the throttling hole makes it possible to enhance the pressure and flow rate of the scavenging gas as compared with the case where a portion in the vicinity of the scavenging inlet port of the scavenging passageway is not throttled. The scavenging gas that has passed through the throttling hole then expands abruptly, thus generating a turbulent state of flow in the gas as it flows through the scavenging passageway and is thereafter blown out from the scavenging outlet port into the combustion actuating chamber. If the scavenging outlet port is also throttled, the flow rate of the gas flow from the outlet port into the combustion actuating chamber is further enhanced.
Accordingly, the atomization of fuel can be promoted, the scavenging efficiency (trapping efficiency) can be improved, and at the same time, the combustion efficiency can be improved. As a result, it becomes possible to obtain a predetermined power with a smaller quantity of fuel, and to effectively minimize the noxious components in the exhaust gas, in particular the total HC, and additionally, the fuel consumption can be improved.
A throttling hole having an open area which is smaller than the cross-sectional area of the scavenging passageway may be formed in a gasket-form of a plate-like member, which is designed to be interposed at the juncture between the cylinder member and the crankcase member so as to throttle a portion of each of the scavenging passageways which is located close to the scavenging inlet port thereof. In such a structure, a separate member is not needed to provide the throttling of the gas. Moreover, it is not necessary to modify the cylinder member or the crankcase member, thus making the present invention very advantageous in terms of manufacturing cost.
Although a mixed fuel comprising a fuel (gasoline) and a lubricant oil is generally employed in the two-stroke cycle internal combustion engine, due to a centrifugal separating effect by the rotation of the engine, in particular, a high speed rotation thereof, a greater portion of the fuel/lubricant oil mixture existing in an air/fuel mixture that has been introduced into the crank chamber is separated from the air and permitted to adhere to the inner wall of the crank chamber. Since the throttled portion (the throttling hole portion) located in the vicinity of the scavenging inlet port of the scavenging passageway constitutes a step portion, most of the fuel which is low in viscosity is allowed to flow into the scavenging passageway together with air, but most of the lubricant oil is caused to remain in the crank chamber due to the blocking effect of the step portion. Therefore, even if the rate of the supply of a fuel/air/lubricant mixture to the engine is reduced, it is possible to secure a sufficient quantity of lubricant oil which is required for lubricating the sliding portions such as the piston, the connecting rod, and the crankshaft, and to avoid a deterioration in the lubricating performance of the engine.
When the attitude of an engine is caused to change (for example, when a chain saw is tilted upwardly) during the idling of the engine, the fuel/lubricating oil mixture which remains inside the crank chamber is permitted to flow excessively into the combustion actuating chamber via the scavenging passageways if the aforementioned step portion is not provided, thereby causing the engine to function poorly and even stop. When the aforementioned step portion is provided as described above, the fuel/lubricating oil mixture which is contained in the crank chamber is impeded from flowing into the scavenging passageways, thereby making it possible to avoid many instances of poor operation of the engine.